Selective Oxidation of Styrene to Benzaldehyde Using Nanobubbles

Recently, the gas-water interface has garnered considerable attention for its ability to remarkably accelerate reactions and trigger impossible reactions in the bulk. Understanding the fundamental role of gas-water interface properties in generating hydroxyl radicals (•OH) is essential to the elucidation of the key reaction mechanism. This study proposes a direct, catalyst-free, and environmentally friendly strategy for the highly selective oxidation of styrene to benzaldehyde in the bulk using nanobubbles. Experimental results demonstrated that the •OH radical, generated during the collapse of nanobubbles, preferentially oxidized the C═C bond of styrene. The hydrophilic benzaldehyde product was spatially separated from the gas-liquid interface, effectively suppressing excessive oxidation to benzoic acid. Isotopic labeling experiments (¹⁸O-enriched water) confirmed that •OH derived from water dissociation served as the primary oxygen source, while dissolved oxygen also participated in the reaction. Density functional theory (DFT) calculations elucidated the mechanism of •OH-induced oxidation of styrene to benzaldehyde and the participation of oxygen significantly lowered the energy barrier. By optimization of ultrasound power and the solvent environment, the reaction kinetics and selectivity mechanisms were systematically investigated. Under the optimized conditions, 24.2% yield and 80.0% selectivity toward benzaldehyde were achieved within 10 h. Furthermore, an ultrasound-laser cycling strategy enhanced reaction efficiency by 1.5-fold compared to continuous ultrasound, attributed to the accelerated collapse of nanobubbles and formation of •OH. This work not only provides a sustainable method for benzaldehyde synthesis but also advances the fundamental understanding of nanobubble-induced interfacial catalysis, offering insights for green organic transformations and scalable applications.